In recent years, light sources using LEDs (Light Emitting Diodes) have attracted attention from the viewpoint of energy saving and reduction of the amount of emitted carbon dioxide. As compared with incandescent bulbs using tungsten filaments, LEDs have longer life and enable energy saving. Because of the convenience, LED illuminations have rapidly developed their market. Many of initial LED illuminations are of a type for obtaining white light by combining an LED that emits blue light and a phosphor that emits yellow light. They can only reproduce unnatural white without warmth. However, the performance remarkably improves as the market of LED products is expanded. As a result of improvements concerning the combination of LEDs and phosphors, various kinds of LED white light sources have been developed.
For example, patent literature 1 discloses a lighting apparatus capable of reproducing five types of white light. LEDs and various phosphors are combined to prepare two types of LEDs which emit white light having a high color temperature and white light having a low color temperature. The light emissions are mixed at various ratios, thereby obtaining five types of white light, that is, incandescent bulb color, warm white, white, daytime white, and daylight white. In an interior illumination, the plural of types of white light are selectively used as needed, thereby obtaining an illumination rendering effect according to a purpose in various scenes of daily life.
On the other hand, recently, illuminations that not only provide a rendering effect but also consider the influences of white light on human bodies have been developed. As a movement, since there is a fear that strong light emitted by a blue LED may have an adverse effect on human bodies, for example, an effect of suppressing secretion of a hormone (melatonin) in human bodies, an illumination that reduces the influence of blue light has been proposed. For example, in patent literature 2, instead of simply reducing the intensity of blue light, LEDs and phosphors having different light emission peaks are combined, and four types of light emission peaks are thus mixed, thereby providing white light in which the influence of blue light is small, and the deviation from the spectral luminous efficiency is small.
As another movement, LED light sources are positively used to recover or build up physical fitness. For example, there is an attempt to directly reproduce sunbeams gentle to human bodies and positively exert influence on the internal clocks of human bodies to promote health. Patent literatures 3 and 7 are inventions directed to a white light source having the same emission spectrum as sunbeams, in each of which sunbeams of different color temperatures are reproduced by the spectrum of black-body radiation having the same color temperature. In the inventions, white illuminations which are approximated to sunbeams having various color temperatures and spectrum shapes, and capable of coping with the rhythm of an internal clock can be obtained. Patent literature 4 is an invention directed to a control apparatus that includes a means for detecting the illuminance or temperature around a human body and, based on the acquired data, adjusts the illumination or air conditioning around the human body in accordance with the biological rhythm. Patent literature 5 is an invention directed to an illumination system using a white light source. This invention is directed to an office illumination or the like which mainly illuminates humans. This system can adjust the color temperature or illuminance of indoor light while detecting a change in outdoor light. A white illumination corresponding to a physiological phenomenon of a human body or a seasonal change can be obtained. Finally, patent literature 6 is an invention directed to an artificial sunbeam system that combines a plural of light emitting diode modules of different color temperatures. This system can reproduce a time-rate change of the color temperature of sunbeams with which a point of a different latitude or longitude on the earth is irradiated.